A bit over 20 years ago I lived on an island that had lousy roads.
Since I was there for a year, I decided to leisurely rebuild the
engine in my car – a 3 litre BMW. I pulled the SOHC six cylinder down and then,
with plenty of time on my hands, decided to clean up the head. Mostly working by
hand with a high speed grinder and sandpaper, I smoothed the ports and slimmed
the valve bosses. I guess that I spent perhaps 50 hours working on that head –
50 hours that was largely a waste of time. Why? Well, with the standard, mild
cam being retained, it’s very unlikely that I gained much extra performance.
In fact, to go further, in most performance car modifications,
re-working the head is about the last thing you do – not the first. However, if
you’re after every bit of engine efficiency, head modifications are still
valid.
Flows
The amount of power that an engine can develop
is dependent upon the mass of air that it can breathe per second. In fact,
engine builders can often estimate quite closely the maximum power that is
achievable from an engine simply by measuring on a flowbench the airflow that
can pass through the cylinder head; some workshops using the following equation
for naturally aspirated engines:
0.43 x maximum port flow (in cfm at 10 inches of
water) x number of cylinders = peak hp
This equation explains why some workshops state
that a particular head “flowed X horsepower”. This is a little deceptive because
the actual airflow efficiency (how much air is consumed for how much power is
developed) varies from engine to engine. When they are tested on an engine dyno,
good engines consume about 1 cfm for each horsepower developed. Also remember
that peak power is not the most important aspect of a road car engine –
average power across the working range is critical.
In the same way, it’s not the flow of peak valve
lift that is important but again it’s average flow across the range of valve
lifts. As one cylinder head porter said: “If we gain 10 cfm at low valve lifts
and lose it at high lifts – great!” This is because the valve spends more time
at lower lifts than full lift, and so low lift flow is more important. However,
gains at low valve lift will not increase the “horsepower figure” that the head
flows!
The restrictions to flow in the cylinder head
comprise both the intake and exhaust ports, and the intake and exhaust valves.
Multi-valve engines have large ‘curtain’ areas – the area found by multiplying
the circumference of the valve by its lift. Thus in many cases, the ports are
the major restricting factor, rather than flow past the valves. Note that this
large valve curtain area means that reverse flow can easily occur when the
piston moves from BDC towards the closing point of the intake valves, causing a
reduction in torque at low engine speeds in engines with lots of valve
overlap.
Flow Benches
As mentioned, a flowbench can be used to
evaluate the volume of air that can flow through the port and past the intake
valve. The flow measurement is carried out at a fixed pressure differential –
normally very small (10, 20 or 28 inches of water). The flow is expressed in
cubic feet per minute (cfm) and is measured at various valve lifts, invariably
expressed in thousandths of an inch.
The larger the “hole” (the cross-sectional area
of the port and curtain area of the valves), the higher will be the airflow
measured on the bench. From this it could be assumed that bigger holes – and so
higher airflow readings – will always be better.
However, this is definitely not the case.
In addition to the mass of fuel/air that is
breathed, the combustion pressure – and so developed torque – is dependent on
the burn characteristics of combustion. Manufacturers build swirl and tumble
into their head designs, resulting in more complete combustion.
Factors determining the swirl and tumble
characteristics include the amount of ‘turn’ that the port has, the angle from
the horizontal at which the port approaches the short side radius, and the shape
of the valves and the chamber. Simply making the port larger without taking into
account these airflow characteristics will not always yield increased engine
power.
Another important factor is that a larger port
area will result in a decreased flow speed. This may result in decreased
cylinder filling at low engine speeds, reducing bottom-end torque. Effectively,
the larger the diameter of the port, the higher peak torque will be moved in the
rev range.
To indirectly indicate port area, port volume is
often measured. With the valve in place and closed, the port volume can be
measured using a burette and kerosene. As a rule of thumb, an increase in 3 cfm
measured head flow should not involve the removal of more than 1cc of material
(ie a 1ml increase in port volume).
Porting
Major porting involves the careful removal of
metal from the passages, enlarging them and often also changing their shape and
even position. We do not recommend that you undertake any major porting of a
modern engine. While there are some professional workshops that successfully
carry out this type of work, these workshops are always equipped with a
flowbench that is used extensively. Further, most of these professional porters
have developed techniques which they will not share with mere mortals like you
or me! Also, even professional workshops can spend many hours porting a head
with only limited success.
An example of only moderate gains
can be seen in the porting carried out on a four-valves-per-cylinder Honda CRX
head. Prior to the porting, a spare Honda head was sectioned so that the porter
could see casting thicknesses – carving into a water jacket isn’t unknown! The
head was then treated to many hours of reshaping of the combustion chamber and
intake and exhaust ports.
So what were the results on the
flowbench? On the intake side, at all valve lifts over 0.275 inches there was a
gain in flow, peaking with a 9 per cent improvement at 0.450 inches lift. On the
exhaust side, flow was the same as stock or lower until a valve lift of
0.250 inches, with the biggest gain being 18 per cent at 0.450 inches lift.
However, as already indicated, the valves are at their maximum lift position for
only a very short time.
In contrast is the porting of a
Suzuki Swift GTi cylinder head. Already an efficient engine developing 58
kW/litre (high for a naturally aspirated engine without variable valve timing),
the G13B four-valves-per-cylinder head responded well to extensive porting. The
intake flow was substantially improved, with flow at 0.100 inches valve lift up
by 28 per cent. This declined to a gain of 14 per cent at 0.250 inches lift,
dropping to only a 1 per cent gain at 0.350 inches lift. On the exhaust side,
flow was very substantially improved all the way from 0.100 – 0.350 inches lift,
being up an average of 36 per cent!
Home Clean-Up
When compared with this type of professional
(and expensive!) porting, how effective is some simple cleaning-up of the heads
by a relatively unskilled amateur? With this approach, obvious casting marks,
steps and jumps in the port walls are smoothed. The port shape is not changed
and the port position remains standard.
While not carried out on a 4-valves-per-cylinder
engine, ‘before’ and ‘after’ figures are available on an AlfaSud 1.5 litre flat
four. In standard form, the 1.5 litre engine develops 75kW (100hp) from its
1490cc – that’s 50kW (67hp) per litre.
Four different porting
modifications were undertaken:
-
The sharp lip on the short radius
from the intake port to the valve was smoothed
-
the rough surfaces of the ports
were smoothed
-
the sharp edges within the
combustion chamber were radius’d
-
the intake manifold was
port-matched
Each cylinder required about five
hours’ work - so 20 hours for both heads.
The flowbench results showed that
the inlet ports improved in flow by an average of 9 per cent, with improvement
recorded at all valve lifts from 0.100 – 0.500 inches. The peak improvement was
12 per cent at both 0.400 and 0.500 inches lift. On the exhaust side, the
average gain was 3 per cent, with the peak improvement being 7 per cent at 0.500
inches lift. Again, at no valve lifts did exhaust flow fall.
Conclusion
The minor cleaning-up of the ports and
combustion chambers is still a viable proposition on current engines, but very
extensive (and expensive) porting can be carried out on sophisticated heads with
no guarantee that the improvement will match the money that is spent. However,
if the head(s) have been disassembled and more power is being sought, it costs
only little to have them tested on a flowbench so that at least informed
decisions about head modification (and cam lift selection) can be made.
